The present invention relates generally to processing systems utilizing heat recovery, and more particularly to a processing system with heat recovery used in the chemical processing of wood chips into pulp for the production of paper.
Many types of processing systems exist for the production of various chemicals, products, consumer goods, etc. One example of such a processing system is a chemical processing system for converting wood chips into pulp used in the manufacture of paper. Wood chips are essentially cellulose bound together by lignin. To convert the wood chips into pulp, the lignin is chemically broken down by subjecting the chips to chemical solutions at high temperatures and pressures in large tanks referred to as digesters. This is a heat-intensive process which consumes much energy.
Previous systems for chemically processing wood chips into pulp have attempted to minimize the energy costs associated with operating the system by using heat recovery to recapture and reuse the heat generated in the process. Such a system is described in an article entitled, "Management System for Liquor Recycling of RDH Digester House," by Ryyn e,uml/a/ nen, et al. That system conserves energy by using multiple tanks for storing and reusing chemical solutions, or liquors, that were originally heated in the digesters. Although desirable to conserve energy, the use of heat recovery systems using multiple tanks complicates the control system responsible for controlling the fluid flows among the tanks and digesters and the fluid levels within the various tanks and digesters. The authors of the above article recognized this problem of coordinating the control of the tanks and digesters, noting that process disturbances would upset the control and require immediate corrections. The ramifications of upsets due to process variations and scheduling problems among a plurality of digesters include reduced pulp output and/or damaged pulp.